To investigate the biological activity of P. judaica and P. officinalis allergen preparations, 36 Spanish patients were included in a biological standardization (BS) trial. For both species the concentration corresponding to 1000 biological units/ml (BU/ml) was found to be about 2 micrograms dry weight/ml (d.w./ml). The allergenic activity per microgram d.w. was about the same as reported for other important weed pollen extracts. The physico-chemical and immunological properties of the purified Parietaria judaica and Parietaria officinalis pollen allergen preparations used for biological standardization were investigated. The protein content was determined to 38% and 23% for P. judaica and P. officinalis respectively, while the carbohydrate content was 25% in both weed preparations. More than 40 antigens and at least 10 different allergens were detected by crossed immuno electrophoresis/crossed radio immunoelectrophoresis (CIE/CRIE) analyses. SDS-gPAGE followed by electroblotting showed important allergens between 10,000 and 75,000 d for both species. The crossreactivity as measured by skin prick test, RAST and CRIE was high.